Because the flight speed of an airplane is very high, its passengers may have to bear a strong downward and forward impact force, when taking an emergency landing. Therefore, most impact absorbers are installed in the passengers' seats to protect the passengers from possible injuries resulting from the impact forces.
The impact absorber mainly utilizes its plastic deformation to absorb the impact energy to which the seats are subjected. There are various kinds of impact absorbers used in all types of airplanes nowadays, such as the inside-out type double-layer tube as shown in FIG. 1, consisting of an inner tube 10 and an outer tube 11. The inner tube 10 is an inside-out double-layer tube body in which the outer edge of the inner tube 10 is firmly connected to the inside edge of the outer tube 11.
As the impact absorber within the seat structure is subjected to bumping, the inner and outer tubes within the impact absorber creates a corresponding stretching in the opposite direction. As shown in FIG. 2, the required energy generated by the inside-out deformation when the inner tube is subjected to stretching, can be shared by the impulse generated by the seats and the passengers, thereby reducing the impact effect and protects the human body from being injured.
FIG. 3 illustrates another prior art. It is an ARA energy absorber which has an inner tube 12 housed in an outer tube 13. The inner tube 12 has a rolling torus 14 firmly secured around its circumference. The rolling torus 14 is tightly and slidably engaged with the inside surface of the outer tube 13. Dotted lines in FIG. 3 show the position of the rolling torus 14 before the impact force is applied. When impact force is applied, the inner tube 12 will be partly pulled out of the outer tube 13. Rolling toms 14 will move axially and exert friction force against the inside surface of the outer tube 13, and thus creating plastic deformation on the outer tube and therefore can absorb the impact forces.
FIG. 4, 5, and 6 depict yet another prior art disclosed in U.S. Pat. No. 5,152,578. It has an outer tube 16 which houses an inner tube 15. At one end of the inner tube 15 located within the outer tube, there is provided a truncated cone attachment 18. The outer tube 16 which tightly surrounds the cone attachment 18 is tapered with a bell-bottom shape opening 17. When impact force is applied, the inner tube 15 will be partly pulled out. The truncated cone attachment 18 will be moved axially and produce plastic deformation on the outer tube (as shown in FIG. 6). Therefore, the impact force will be absorbed. FIG. 5 shows one of the possible embodiments. It has four convex arcs around the circumference of the cone attachment 18. Only the convex arc areas can have contact with the outer tube and create the plastic deformation. Changing the length of the convex arc can change the capability of absorbing the impact force. However, the maximum length of the convex arcs is the circumferential length of the cone attachment 18. Therefore, it has a maximum limitation in the impact force absorbing capability.
There are three disadvantages according to the prior art of impact absorbers:
1. Their capability to absorb the impact force is highly dependent on the material properties (e.g. mechanical strength, rigidity, etc.), and the dimensional tolerance of the parts involved. The exact amount of impact force they can absorb is difficult to determine or control. Maintaining consistent product quality becomes a serious manufacturing problem. PA0 2. On the design work, the manufacturer has to obtain the amount of impulse absorption capability by means of experimentation. When it comes to designing the change of impulse absorption capability, the manufacturer has to repeat the experimental work until the required new design is obtained. PA0 3. Although the U.S. Pat. No. 5,152,578 disclosure can partly solve the problems set forth above, its capability to absorb the impact force is limited by the total circumferential length of the cone attachment.